[Bug 111475] Brightness control inversion from VBT table is not taken into account in Ubuntu 18.10

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111475

Lakshmi <lakshminarayana.vudum at intel.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jani.nikula at intel.com
             Status|NEEDINFO                    |NEW

--- Comment #7 from Lakshmi <lakshminarayana.vudum at intel.com> ---
(In reply to Szymon from comment #4)
> I'm not sure what do you exactly mean by "BIOS enable the display". For sure
> LVDS display is switched on and I can see the screen. I assume BIOS enabled
> it.
> 
> I provided requested files as attachments.
> 
> Can you point Intel's document number which describes the content of
> register which should be set by BIOS?
> 
> P.S. I'm running Ubuntu from USB stick (without installation).

(In reply to Szymon from comment #5)
> Today I performed another tests a here are the results:
> 
> 1. Confirmed that Linux does not take VBT backlight active low bit into
> consideration. If this is the bug that is to be decided by Linux graphic
> driver developers. NOTE: Windows graphic driver takes this setting into
> consideration.
> 
> 2. Once BIOS writes 0xC8250 MMIO register with bit 29, backlight logic is
> inverted correctly. However 10% minimum brightness level defined in VBT
> table is NOT taken into account in this case, i.e. Linux brightness slider
> set at maximum produces complete darkness instead of 10% brightness (as set
> as minimum).
> 
> 3. In comparison to point 2: Once backlight logic is inverted by kernel boot
> parameter acpi_backlight=vendor i915.invert_brightness=1, slider at max
> still respects minimum brightness level 10% defined in VBT.


Jani, any further comments?

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